So I recorded a random thunderstorm in Barcelona with my ZOOM H1, and when I analyzed the audio I noticed some activity in the higher frequencies during 20 minutes. I pitched them down 18 tones to make them audible - here's the result.

bulbastre this is a very interesting sound and a very interesting dare to try to find out what this is.So, continuing our private message discussion, i proposed that the initial part of the sound (the burps and bursts) could be electromagnetic interference caused by the storm itself. However, the most puzzling to me is the second sound (the drone). Not only i don't have an idea yet of what this might be, but also it does not at all overlap with the first sound.

Anyone out there with more experience in outside recording got any comments? Could these sounds be bats?

Also bulbastre, more information please: was this recorded by day or night? From the sample description i take it it was recorded in a buit up area. Any obvious structures nearby such as power cables, transformers, rf antenas, mobile phone posts?What about your own gear? Any laptop, ipod, mobile phone, etc nearby (say, within 2 or 3m)?

Could you / have you returned to exactly the same location and tried recording the sound again?

My window faces to the intern "garden" of the neighbourhood (see the map) - the direction where I recorded. There's usually children playing around, but it was a rainy day, so it was empty. RF anthennas... Maybe, since this parts of the neighbourhood are usually where the satellite antenas are. However, nowadays it's difficult to see this kind of antenas in Spain, since Digital Television and Cable Television have taken the market.

There is a major street -Numancia- on the other end of my house, but no traffic sound can be heard from where my room is. Also, I placed the recorder OUTSIDE my room, separated for a double-glass window that isolates sound and temperature very well. It is impossible for the recorder to pick up any sound coming from my room. In fact, I was probably listening to music or working with movie audio, and neither of both can be heard during the whole recording.

In Barcelona sometimes you feel and hear the underground passing by. Although there's a train and metro station near, I never in my life felt or heard them. Never. Besides, metro and trains deliver low frequencies, not high ones.I need to recheck the original audio, to see if there's some low frequency activity accompanying the high ones, but I haven't noticed anything so far. I'm uploading the original audio, so everyone can research and experiment.

And YES! There's more of this mysterious sound in other parts of the recording. I recorded 8 hours of audio, and I only checked 2. The sound appears twice, but I'm sure it's somewhere else in the 6 hours I've not checked yet.

Hi, thank you for providing the additional info.Just bear in mind that if the sound is the result of electromagneric interference, it may not necessarily link to something you can hear. Think of the interference that mobile phones generate on car stereos, for example.

Trains could generate such interference. The moving connection on the overhead wire sometimes generates sparks, and those generate wideband radio noise bursts. However, the subway travels several meters underground, and some meters of soil should shield out any electromagnetic noise.As for you working in your room during the recording, wi-fi internet can generate the same sort of interference as mobile phones and it can travel through walls or double glass. Altough the sounds on this recording do not seem like mobile or wi-fi interference...

Another interesting idea tonexplore is that sound travellinga great distance through air looses the low frequencies first...

This is confusing (to me). As it stands it unavoidably makes me think of flying saucers, but this kind of associations of ideas in fact hinder the mental process of imagining a real source for the sound. Even if inaudible, the untouched file would be more useful at this. Any chance that you upload the original file, part of it?

AlienXXXTrains could generate such interference. The moving connection on the overhead wire sometimes generates sparks, and those generate wideband radio noise bursts. However, the subway travels several meters underground, and some meters of soil should shield out any electromagnetic noise.

During 20 minutes? The bursts go at first, and they look to regular to me. The second part is constant.

AlienXXXAs for you working in your room during the recording, wi-fi internet can generate the same sort of interference as mobile phones and it can travel through walls or double glass. Altough the sounds on this recording do not seem like mobile or wi-fi interference...

I ALWAYS have wifi on, and the sound comes in and out abrupty. Also, I have recorded in my room several times before. Not eight hours straight, ok, but several times. With cables, external soundcard, etc. the whole deal, and I never heard it.

AlienXXXAnother interesting idea to explore is that sound travellinga great distance through air looses the low frequencies first...

Isn't it the opposite? Like when sound traverses a wall or any fisical object. It looses high frequencies. I think it also works like this with radio signals - you first loose the higher ones.And even taking that it comes from afar... Shouldn't it carry any reverb with it?

My first attemp to download the file failed. I only managed to download it yesterday before i went to bed. Now i will be able to look at it and listen very carefully in a sound editor, rather than using freesound's sound preview.

When sounds travel long distances through air they looselow freqfirst. When sounds travel through walls they loose high freq first. The physics of the two fenomena are different: one is the long distance travel through a constant medium; the other involves changing the transmitting medium from air to wall and back to air (and repeated again if going through more than one wall).That is why distant firewodks always sound like feeble "pops" (no low freq) and your neighbour's stereo always sounds like "boom-bomm-boom"

Radio freq does not obey the same rules. Also, radio freq interference can cause different effects depending on what is interfering with what. The 50Hz hum from the mains translates directly into sound as it is at an audible freq. But when your friends mobile interferes with you car radio you do not hear his conversation blended with the music... All you get is that characteristic bzzzt...bzzzt...bzzzt sound.

I will make some more comments once i have gone through the file in detail.Did you have the auto-volume on during this recording?

Ooooooook.These two last comments seem very revealing even if I don't understand a single word you're saying.

What I understand is that:1. It might come from some fluorescent light provided they somehow might be affected by thunderstorms. I don't know what's EMI. I also don't see a direct relation between working at a different voltage and emiting a high frequency sound.2. The first part of the sound might come from an error of the recorder itself. Aren't these two theories excluding?

What I gather is that:I should record during good weather.a- If the sound continues to appear, 2 is more likely.b- If the sound ceases, 1 is more probable.

As a side note I would be very interested to know what you have used to demodulate the sound (that is an effect/sound processing I have been trying to find for some time).

However, I am not sure I understand how or why it is relevant for this case.My hipotesis is that the "sounds" that bulbastre recorded are ElectroMagnetic Intereference (EMI), and therefore they are not sounds at all. They are a radiofrequency signal that has interfeered with his recording equipment. - If this is the hipotesis that you are working from too, can you explain why "demodulation" is important?Also, for sure when demodulating a signal the carrier frequency used must be very important to the resulting sound. So why the choice of 11.8KHz?

There is, of course, another hipothesis - which would be to say that the sounds bulbastre recorded are indeed very high frequency sounds. In which case we should be looking for a source of sound rather then EMI.

1] Swith on your recorder and microphone on normal level. 2] Pack them in aluminium foil to protect from electronic interference.3] Bury the equipment on and under pillows and blankets in a closet in a quiet room. Make a recording of one hour. Then check if the sound is still there.if YES: recorder in damaged.if NO: then came the sound from outside. It could have been anything. A Gost??

I have looked at the geomap for your location but I could not tell if there are airconditioning units at the top of the buildings. These could be a source of both sound and/or EMI.

Another point. As I said, i have downloaded your first file (the re-pitched sound) and could now listen more carefully on a sound editor.

The second sound (which I have called the 'drone')also appears at the start of the file. In fact it is the first sound that can be heard. It lasts about 10s. Before this sound ends, the lower freq bursts start. Then the drone stops. There is another short 'drone' sound lasting approx 1s, at around 16s.Again at 32s there are a set of several short 'drone' sounds.So basically, the drone sound appears throughout the recording, only initiallyis is discontinuous and then it becomes one continuous sound.The discountinuous sounds are "grouped" in events... within each even they become progressively weaker. Then another event starts again with various discountinuous sounds becoming weaker and. Overall, the pattern is that each event starts at a lower volume than the one before, until eventually the discountinuous sounds disapear and the continuous 'drone' starts.

For me there are some important findings here:1) the 'drone' sound is present throughout the recording, althoug initially it is discountinuous.2) the higher pitched drone can overlap with the lower pitch sound which is only present in the "bursts" section of the recording. For example at 16s, several times between 1m27s-1m37s, etc)

So... what could it be...The bursts could be some failing electronics. The suggestion of a flickering fluorescent lamp is an interesting one. Then the lanp stabilizes and we get the continuous sound.There would have to be 2 separate circuit elements producing the sound, as there are high and low pitch bursts and sometimes they overlap.Don't know enough about electronics to discuss if the flickering fluorescent light could produce this type of interference.

If I am honest, I am more inclined to think the interference is caused by several wi-fi devices talking to each other. The various bursts could be the call and response signals and the keep-connection-alive signals... The continuous 'drone' could be an actual data transmission.

In both cases, however, I struggle to explain why the volume of the bursts does not remain more or less constant?Why do the bursts become lower in volume within each event (or group) ?And why does each event tend to be lower in volume that the one before ?

1] Swith on your recorder and microphone on normal level. 2] Pack them in aluminium foil to protect from electronic interference.3] Bury the equipment on and under pillows and blankets in a closet in a quiet room. Make a recording of one hour. Then check if the sound is still there.if YES: recorder in damaged.if NO: then came the sound from outside. It could have been anything. A Gost??

CAREFULL - don't throw away the recorder just yet!Make sure you try it with a difference memory card first!!! it could be the card itself.In fact, i think the Zoom H1 has a small internal memmory, so you may be able to do a short recording with no card at all.Or borrow a card from someone... these are common nowadays in cameras, mobile phones, etc...

I have one as well, and I am quite pleased with it. Although my recording requirements are perhaps fairly modest. And most of the time I do not record outside (need to bui the foam wind protector)

My experience with electromagnetic interfeerence is mostly from deliberate recordings I have made using a few coil sensors (some home made).Initially I plugged these directly onto my soundcard or laptop mic socket.Now I have used them with the Zoom H1 too, and even made some outdoors recordings with them... Had a particluarly interesting recording made when aboard an electric train... If I can find it I will post on Freesound.